Embedded magnet rotors of known type, used especially in brushless motors, usually consist of a laminated core, that is to say, a core made up of a plurality of thin metal laminations and having a principal axis that coincides with the axis of rotation of the motor.
The rotor normally has a plurality of longitudinal slots, parallel to the principal axis, and a central hole or opening, also parallel to the principal axis, for accommodating the magnets and a drive shaft, respectively.
The rotor is mounted in a stator equipped with magnetic poles which, when powered, generate a magnetic field that interacts with the magnets and causes the rotor to turn.
One disadvantage of motors that use rotors of this kind is due to the magnetic attraction between the magnets in the rotor and the poles on the stator giving rise to what is commonly referred to as “cogging torque” or “detent torque”
Cogging torque causes non-uniformity in the angular speed of the rotor, which tends to rotate in a series of small “jerky” movements instead of smoothly and continuously.
Embedded magnet rotors are also difficult to balance and assemble, especially when the magnets are inserted into the longitudinal slots.
The inside dimensions of the slots are slightly larger than the outside dimensions of the magnets so that the latter can be inserted into the slots.
The result is that in prior art rotors, however small these dimensional differences between slots and magnets may be, the magnets are not held firmly in place and the centrifugal force created by the rotational motion pushes them against the radially outermost wall of the slot.
That means the weight distribution and air gap shape when the rotor is at rest differ from the weight distribution and air gap shape when it is rotating.
In other words, the position of the magnets relative to the rotor core changes when the motor is started, making it very difficult to optimise flows and geometry in the motor design.
Optimisation of geometry is also made more difficult by concentricity errors in the different sheets making up the rotor due to the fact that the sheets are cut at different times. These errors cause rotor unbalance.